NAME

VERSION

version 2.2009

WHAT IS A ROLE?

A role encapsulates some piece of behavior or state that can be shared between classes. It is something that classes do. It is important to understand that roles are not classes. You cannot inherit from a role, and a role cannot be instantiated. We sometimes say that roles are consumed, either by classes or other roles.

Instead, a role is composed into a class. In practical terms, this means that all of the methods, method modifiers, and attributes defined in a role are added directly to (we sometimes say "flattened into") the class that consumes the role. These attributes and methods then appear as if they were defined in the class itself. A subclass of the consuming class will inherit all of these methods and attributes.

Moose roles are similar to mixins or interfaces in other languages and are based on the original concept of Traits for the Smalltalk-80 dialect Squeak.

Besides defining their own methods and attributes, roles can also require that the consuming class define certain methods of its own. You could have a role that consisted only of a list of required methods, in which case the role would be very much like a Java interface.

Note that attribute accessors also count as methods for the purposes of satisfying the requirements of a role.

It's possible to compose existing roles into new roles. For example, we can have a HandleWithCare class which applies both the Breakable and Package roles to any class which consumes it:

packageHandleWithCare;useMoose::Role;with'Breakable','Package';

REQUIRED METHODS

As mentioned previously, a role can require that consuming classes provide one or more methods. Using our Breakable example, let's make it require that consuming classes implement their own break methods:

If we try to consume this role in a class that does not have a break method, we will get an exception.

You can see that we added a method modifier on break. We want classes that consume this role to implement their own logic for breaking, but we make sure that the is_broken attribute is always set to true when break is called.

Roles Versus Abstract Base Classes

If you are familiar with the concept of abstract base classes in other languages, you may be tempted to use roles in the same way.

You can define an "interface-only" role, one that contains just a list of required methods.

However, any class which consumes this role must implement all of the required methods, either directly or through inheritance from a parent. You cannot delay the method requirement check so that they can be implemented by future subclasses.

Because the role defines the required methods directly, adding a base class to the mix would not achieve anything. We recommend that you simply consume the interface role in each class which implements that interface.

CONSUMING ROLES

Roles are consumed using the with function.

Most of the time, you should only use one with, even if you are consuming multiple roles. If you consume roles using multiple with statements Moose cannot detect method conflicts between those roles.

Roles can be consumed by classes or by other roles. When a class consumes a role which in turn consumes other roles, the class gets all of the roles applied at once.

Required Methods Provided by Attributes

As mentioned before, a role's required method may also be satisfied by an attribute accessor. However, the call to has which defines an attribute happens at runtime. This means that you must define the attribute before consuming the role, or else the role will not see the generated accessor. These attributes are Moose Attributes.

In general, we recommend that you always consume roles after declaring all your attributes.

It may also be the case that a class wants to consume two roles where one role has an attribute providing a required method for another. For example:

packageBreakable;useMoose::Role;requires'stress';########packageStressable;useMoose::Role;has'stress'=>(is=>'ro',isa=>'Int',);########packageCar;useMoose;# XXX - this will not workwith'Breakable','Stressable';

However, this won't work. The problem is that the accessor methods created for the stress attribute won't be present in the class when the required method checks are done.

There are two possible workarounds. The recommended one is to use "stub" subroutine(s) in the role providing the accessor(s):

The sub stress; line is called a "forward" declaration in the Perl documentation. It creates what is called a "stub" subroutine, a declaration without a body. This is good enough to satisfy the required method checks done by Moose. The stub will not interfere with the creation of a real subroutine later.

The other alternative is to use two separate calls to with in the consuming class:

packageCar;useMoose;# Not recommendedwith'Stressable';with'Breakable';

Each with is run as it is seen. The first call will consume just the Stressable role, which will add the stress attribute to the Car package, which in turn will create an accessor method named stress. Then when the Breakable role is consumed, the method it requires already exists.

However, as mentioned earlier, multiple with declarations are not recommended, because method conflicts between the roles cannot be seen. In the example above, if both Stressable and Breakable contained methods of the same name, what would happen is that the version in Stressable would silently override the one in Breakable.

USING METHOD MODIFIERS

Method modifiers and roles are a very powerful combination. Often, a role will combine method modifiers and required methods. We already saw one example with our Breakable example.

Method modifiers increase the complexity of roles, because they make the role application order relevant. If a class uses multiple roles, each of which modify the same method, those modifiers will be applied in the same order as the roles are used:

Assuming that the new ExplodesOnBreakage role also has an after modifier on break, the after modifiers will run one after the other. The modifier from Breakable will run first, then the one from ExplodesOnBreakage.

METHOD CONFLICTS

If a class composes multiple roles, and those roles have methods of the same name, we will have a conflict. In that case, the composing class is required to provide its own method of the same name.

packageBreakdancer;useMoose::Role;sub break {}

If we compose both Breakable and Breakdancer in a class, we must provide our own break method:

The excludes parameter prevents the break method from being composed into the FragileDancer class, so we don't have a conflict. This means that FragileDancer does not need to implement its own break method.

This is useful, but it's worth noting that this breaks the contract implicit in consuming a role. Our FragileDancer class does both the Breakable and BreakDancer, but does not provide a break method. If some API expects an object that does one of those roles, it probably expects it to implement that method.

In some use cases we might alias and exclude methods from roles, but then provide a method of the same name in the class itself.

OVERLOADING

When a Moose role uses overloading, that overloading is composed into any classes that consume the role. This includes the setting of the fallback value for that role's overloading. Just as with methods and attributes, when a role consumes another role, that other role's overloading settings are applied to the role.

Just as with methods, there can be conflicts with overloading implementations between multiple roles when they are all consumed by a class. If two roles both provide different overloading implementations for a given operator, that is a conflict. If two roles both implement overloading and have different fallback values, that is also considered a conflict. These conflicts are detected when multiple roles are being composed into a class together.

When a role consumes another role, the consuming role's overloading fallback and operator implementations silently "win" the conflict.

ROLE EXCLUSION

A role can say that it cannot be combined with some other role. This should be used with great caution, since it limits the re-usability of the role.

packageBreakable;useMoose::Role;excludes'BreakDancer';

ADDING A ROLE TO AN OBJECT INSTANCE

You may want to add a role to an object instance, rather than to a class. For example, you may want to add debug tracing to one instance of an object while debugging a particular bug. Another use case might be to dynamically change objects based on a user's configuration, as a plugin system.

The best way to do this is to use the apply_all_roles() function from Moose::Util:

This function can apply more than one role at a time, and will do so using the normal Moose role combination system. We recommend using this function to apply roles to an object. This is what Moose uses internally when you call with.

Handling required attributes for roles.

Application of some roles will require additional parameters being specified to satisfy them, for example:

{packageCar;useMoose;}{packageBreakable;useMoose::Role;has'breakable_parts'=>(is=>'ro',required=>1);}my$car=Car->new;# next line dies with: Attribute (breakable_parts) is requiredapply_all_roles($car,'Breakable');

This will require passing the additional parameters at application time as follows: